The Association of Maximum Service Television (MSTV) has asked the FCC to clarify technical details of its Notice of Proposed Rulemaking (NPRM) that would allow unlicensed devices to operate on vacant TV channels. MSTV said it is planning to do laboratory tests, an assessment of available spectrum for unlicensed devices and "other tests that are deemed appropriate to develop a technically sound and complete record in this proceeding, such as an evaluation of the feasibility and practicality of unlicensed equipment that would provide the interference protection necessary to safeguard the integrity of the broadcast television service." However, the NPRM does not include all the technical data needed for MSTV or any other party necessary for such an evaluation.

MSTV's Request for Clarification said that while MSTV is willing to start the study and take the risk that "its preliminary findings may rest on an incomplete understanding of the NPRM's parameters," it requests the FCC provide clarification on the issues it raises as soon as possible.

Some of the information MSTV is seeking includes the minimum and maximum operating bandwidth of the unlicensed devices and how they would be channelized. MSTV said, "Lacking clarification from the Commission, MSTV will assume that these fixed devices will operate on multiple channels and will be placed in locations where they have to meet all the required D/U ratios." MSTV also asked what type of modulation would be allowed for the unlicensed devices, noting, "depending on the modulation-type used, interference from a device may appear as noise-like interference on a television set, a band across the television screen, or a total elimination of the television picture and/or sound." "Without specification by the commission regarding modulation type, MSTV will have to assume a single modulation-type that, in its good faith judgment, best approximates the likely character of the unlicensed devices that would operate in the television spectrum. Using a single-modulation type would limit MSTV's findings to the chosen modulation scheme and may not necessarily be appropriate to the actual modulation-type eventually used by the unlicensed devices."

MSTV's Request for Clarification notes that while the NPRM specifies F (90, 90) curves to determine the desired signal level of the unlicensed device operation inside a TV station's adjacent contour, it does not provide any guidance on the correction factor to be used to adjust the FCC's F (50, 50) or F (50, 90) curves to obtain an F (90, 90) field strength. MSTV said without guidance from the commission, they would apply a 7.1 dB correction factor to the F (50, 90) curves for UHF and VHF. Questions also exist about how to determine undesired signal level. The NPRM proposes use of F (50, 50) curves for this, but bases the height on antenna height above ground, rather than the height above average terrain (HAAT) specified in the F (50, 50) curves. As imagined, for stations not located on flat terrain there will be large differences in predicted field strength depending whether HAAT or height above ground is used. The signal from an antenna ten feet above ground will be a lot different if it is on a hilltop instead of in a valley.

Finally, MSTV mentions that the NPRM proposes to allow manufacturers and distributors of unlicensed devices to calculate undesired signal levels using "other appropriate models" beside the broadcast F (50, 50) curves. Without knowing the parameters for these unspecified models, it is impossible to assess interference risks. See the MSTV Request for Clarification for additional information.